1. Field of the Invention
The invention relates to a light pick-up device of an opto-electronic scanning element for transparency and opaque masters in either a flat bed or a drum scanning device.
2. Prior Art
Flat bed or drum scanning devices are employed, for example, in facsimile transmission technology. A light beam scans a master or original, which is to be reproduced point-by-point and line-by-line. The scanning light, which is modulated by the master, is directed upon an opto-electronic scanning element, which converts it into an image signal. The image signal is transmitted over a transmission channel to a facsimile recording device, which records a reproduction of the master or original.
The masters to be reproduced may be type masters, rastered or unrastered image masters, or masters, which are assembled of type and images and are so-called paste-up montages.
Traditional opto-electronic scanning elements essentially consist of at least one light source, which provides a punctiform illumination of the master, a lens, an opto-electronic transducer for generating the image signal from a modulated light received from the master, and a diaphragm disposed between the lens and transducer. Whereas the light source produces a relatively large illumination spot on the master, the diaphragm aperture determines the size of the actual, smaller scanning spot and the scanning fineness or resolution as well, because the scanning spot on the master is sharply imaged in the diaphragm by means of the lens. For the purpose of a sharp imaging of the scanning spot, even in case of thickness fluctuation of the masters, which may occur with either paste-up montages, or with non-uniform color application, etc., the lens must have a great depth of field since a readjustment of the focus of the spot during the scanning is involved. However, a great depth of field for the lens can in turn only be achieved with a small aperture angle for the lens.
Masters will modulate the scanning light by diffusely reflecting it at a large solid angle. With scratches in the master and with so-called shadow edges in the case of the paste-up montages, the intensity distribution of the modulated light, which is reflected back by the master, is asymmetrical within the solid angle. This asymmetrical intensity distribution causes scanning errors in the case of a nearly punctiform brightness measurement. In order to reduce such scanning errors, the modulated light must be collected, i.e., the scanning element must pick up the asymmetrically modulated light over a solid angle which is as great as possible, and a large aperture angle of the lens is required for this purpose.
Thus, in a traditional scanning element with diaphragm and lens, a demand for a great depth of field at a small aperture angle is opposed by a demand for a large aperture angle in order to reduce these scanning errors. Therefore, a compromise must be made between these two demands and this is viewed as a considerable disadvantage.
In order to reduce the scanning errors, light pick-up devices in the form of hollow bodies are known from the U.S. Pat. Nos. 4,080,634 and 3,603,730 in which the modulated light coming from the master is collected and supplied to light-sensitive surfaces and the disclosures of these two patents are incorporated by reference thereto. In the case of the device in U.S. Pat. No. 4,080,634, the light-sensitive surface is a photodiode strip, and, in the case of the U.S. Pat. No. 3,603,730, the entire inside surface of the hollow body is designed as a light-sensitive surface.
These known light pick-up devices have the disadvantages that the light-sensitive surfaces (photodiodes) are relatively insensitive and slow, and as a result, the device can achieve only a low scanning rate. A higher scanning rate could be achieved, for example, by employing fast photomultipliers. Given the known light pick-up devices, the disposition of a multitude of such photomultipliers instead of the light-sensitive surface would be too involved or, would hardly be possible for reasons of space requirements. A further disadvantage of the known light pick-up device is that they are only suitable for scanning opaque masters in a flat bed scanning device and are not suitable either for transparency masters or for drum scanning devices.